This invention relates in general to analog-to-digital converters and in particular, to current-input, autoscaling, dual-slope analog-to-digital converters ("A/D converters").
In certain applications, it is desirable for A/D converters to measure inputs over as wide a range as possible. For example, in potentiostat applications such as anodic stripping voltammetry and square wave voltammetry analyses with microelectrodes, currents to be measured can range from the picoampere level to over 10 .mu.A.
FIG. 1 illustrates, as an example, a block diagram of a conventional voltage-input, autoscaling A/D converter 3 commonly used to measure a process parameter over a wide input voltage range. The autoscaling A/D converter 3 employs a scaling function by using a programmable gain, front end amplifier 5 to scale the input voltage V.sub.in to a fixed binary range (e.g., "full-scale") of an A/D converter circuit 7. This results in a floating-point style conversion, where the A/D converter circuit 7 determines the mantissa and the gain setting of the front end amplifier 5 determines the exponent. A controller 9 sets the gain setting of the programmable gain amplifier 5 such that the gain setting is increased for small digital outputs relative to the fixed binary range of the A/D converter circuit 7, and reduced for large digital outputs relative to the fixed binary range of the A/D converter circuit 7.
The front end or programmable gain amplifier 5 is conventionally implemented using a network of resistors (not shown) that can be individually selected to obtain a desired gain setting. To minimize linearity errors at the points where the gain setting changes, the resistors are conventionally either precisely matched discrete resistors, or on-chip laser-trimmed resistors. Both types of resistors significantly add to the cost associated with such autoscaling A/D converters.